Use ConfigLoader.default_configuration for default

When the config file path is same as the default config file,
use `ConfigLoader.default_configuration` instead of create a new config.

Because some plugins add their default config values to
`ConfigLoader.default_configuration`. So the Cops of the plugins expect
that default config values exists.

Fix rubocop/rubocop-rails#127

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [rubocop-hq/rubocop-rails#127](https://github.com/rubocop-hq/rubocop-rails/issues/127): Use `ConfigLoader.default_configuration` for the default config. ([@hanachin][])
+
 ## 0.79.0 (2020-01-06)
 
 ### New features
@@ -4320,3 +4324,4 @@
 [@mangara]: https://github.com/mangara
 [@pirj]: https://github.com/pirj
 [@pawptart]: https://github.com/pawptart
+[@hanachin]: https://github.com/hanachin

lib/rubocop/config_loader.rb

@@ -83,9 +83,9 @@ def configuration_file_for(target_dir)
       end
 
       def configuration_from_file(config_file)
-        config = load_file(config_file)
-        return config if config_file == DEFAULT_FILE
+        return ConfigLoader.default_configuration if config_file == DEFAULT_FILE
 
+        config = load_file(config_file)
         if ignore_parent_exclusion?
           print 'Ignoring AllCops/Exclude from parent folders' if debug?
         else

spec/rubocop/cli/cli_options_spec.rb

@@ -508,7 +508,7 @@ class SomeCop < Cop
       home = File.dirname(File.dirname(File.dirname(File.dirname(__FILE__))))
       expect($stdout.string.lines.grep(/configuration/).map(&:chomp))
         .to eq(["For #{abs('')}:" \
-                " configuration from #{home}/config/default.yml"])
+                " Default configuration from #{home}/config/default.yml"])
     end
 
     it 'shows cop names' do

spec/support/cli_spec_behavior.rb

@@ -11,6 +11,7 @@ def abs(path)
 
   before do
     RuboCop::ConfigLoader.debug = false
+    RuboCop::ConfigLoader.default_configuration = nil
 
     # OPTIMIZE: Makes these specs faster. Work directory (the parent of
     # .rubocop_cache) is removed afterwards anyway.